The objectives of this proposal are centered around the evaluation of the evolutionary basis of immunity. Several immunological parameters will be considered: the immunogenetic and histo-pathologic basis of allograft rejection, graft-vs-host reactivity, immunological maturation and tolerance; the study of humoral and cellular kinetics of primary and secondary humoral responses to soluble and cellular antigens by histological and ultrastructural studies paralleled with serological and basic immunochemical studies; the effects of environmental temperature on cellular and humoral immunity as a means of controlling and studying discrete events in each type of response; the surface topography and antigen and mitogen-binding capacities of immuno-competent cells derived from central and peripheral lymphoid organs by ultrastructural analysis; the functional aspects of the antigen-trapping mechanism of the spleen in primary and secondary immune responses by histochemical means; a phylogenetic survey of the lymphoid organs in representative species of various animal classes, so that the above objections can be expanded to cover more of the phylogenetic tree. The comparative evaluation of the various aspects of immune responsiveness would lead to a clearer understanding of the more complex mammalian immunological systems.